1. FIELD OF THE INVENTION
The invention relates to a stopper for a tap hole of a steel converter, comprising a plug which is dimensioned so that it fits at least partly into the tap hole. The invention also relates to a method of operating a steel converter having a tap hole, in which such a stopper is removed from the tap hole by the action of the molten steel on tipping of the converter.
2. DESCRIPTION OF THE PRIOR ART
In order to tap the steel refined in the converter the converter is tipped. Unless the tap hole is blocked, tapping will start with the casting of a quantity of slag, the so-called initial slag, which floats on the steel. The quantity of this slag which is tapped depends on the speed at which the converter is tipped and the size of the tap hole.
The steel following the slag through the tap hole is still unkilled which means that it contains a lot of unbound oxygen. In order to obtain killed steel the oxygen must be bound, for which purpose aluminium is added. The effect of the aluminium is reduced if this also reacts with the oxygen in the initial slag cast. The steel quality may be reduced by an increase in the phosphorus and sulphur content in the steel, these elements being released in the reaction of aluminium and slag. These reactions also lead to unpredictable yields of the additives used during steel preparation. In addition the slag can cause unwanted inclusions in the steel. For all these reasons tapping of the initial slag with the steel must be avoided as much as possible.
In order to prevent these problems it has been proposed that the tap hole be sealed from outside the converter with a stopper. This stopper comprises a plug in the form of a truncated cone, the base circle of which is bigger than that of the tap hole, and a head circle which is smaller than that of the tap hole. The plug fits partly in the tap hole. See NL-A-No. 7612060 and U.S. Pat. No. 4,399,986 for example.
The stopper of NL-A-No. 7612060 is fixed in the tap hole by external fittings e.g. by a spring device located outside the converter which can exert a force on the head plate of the stopper. This head plate is made of steel with a thickness selected such that on tipping the converter it can hold back the converter slag running into the tap hole, but as a result of the load from the following steel melts within a few seconds.
The stopper of NL-A-No. 7612060 also comprises barbs around the circumference of the conical surface which serve to prevent the plug being pushed into the tap hole and which barbs probably also have a function of avoiding the conical plug tilting in the tap hole. This stopper has a number of disadvantages. When the converter is tipped a quantity of slag runs into the tap hole, and this slag has to be pushed out by the molten steel before the sealing effect of the stopper is lost. This does place requirements on the minimum holding time of the stopper. Also because the length of the tap hole is about 1.5 meters and the diameter 0.2 meters, so that the column of slag in the tap hole weighs about 700 kg, the ferrostatic pressure in the hole must also be taken into account.
In addition during the blowing process in the converter, the tap hole becomes silted up with slag, and it also happens that the tap hole is subject to wear near the internal wall of the converter as a result of direct contact with the blowing process.
In addition this known stopper can only be used with the external spring device which is expensive and requires special measures in order for it also to be used when the converter is tipped. In addition special measures are required to prevent the molten steel coming into contact with the spring device at the start of tapping of the molten steel, which would make reuse of this spring device impossible.